This invention relates to bifocal contact lenses and particularly to a bifocal contact lens structure that eliminates any vision jump between lens powers. The invention also encompasses a method and apparatus for manufacturing bifocal contact lenses, particularly, bifocal contact lenses with no vision jump between lens powers.
Bifocal lenses provide vision correction for both distance vision and up-close or near vision. As the name bifocal suggests, bifocal lenses provide the two different types of vision correction with two separate lens sections, each having a different focal length. In bifocal contact lenses, each lens must remain properly oriented on the wearer's cornea in use with the distance vision section at the top of the cornea and the near vision section at the bottom of the cornea.
There are two general categories of bifocal contact lens designs. In one bifocal contact lens design the lens has a non-uniform index of refraction. A lens of this design type is commonly referred to as a fused or two-piece lens. U.S. Pat. No. 3,662,040 to Urbach shows a two-piece or fused bifocal contact lens comprising two materials with different indices of refraction. The Urbach bifocal contact lens structure includes a lens base formed from one lens material. A lens material having a different index of refraction is fused to the lens base material in an opening formed in the lens base. The two types of material produce the desired two different vision correction powers.
The other general bifocal contact lens design, the one-piece design, is produced from a single material with a uniform index of refraction and produces the different correction powers with a non-uniform curvature on the outer or anterior surface of the lens. U.S. Pat. No. 5,074,082 to Cappelli and U.S. Pat. No. 4,938,583 to Miller show one-piece bifocal contact lens designs. In both the Cappelli and Miller lens designs, the anterior surface of the lens includes two separate sections, each with a different radius of curvature with respect to the curvature of the lens inner or posterior surface. The two sections with dissimilar curvature produce the desired two lens powers.
Both the fused and prior one-piece bifocal lens designs had drawbacks that prevented their wide spread acceptance and use. Both designs relied only upon asymmetry built into the lens shape or weight distribution to maintain proper orientation of the lens on the patient's cornea. The two-piece or fused bifocal contact lenses were relatively expensive to manufacture and were also thicker and, therefore, less comfortable to wear. Also, the two-piece design limited the types of materials that could be employed in the lens. Furthermore, there was the risk that the fused portion of the two-piece design could detach from the base portion of the lens.
The one-piece bifocal contact lenses were thinner and generally less expensive to manufacture. However, the prior one-piece lens designs produced a vision jump or vision distortion at the junction between the two different lens powers. The vision jump or distortion between the lens powers was distracting to the wearer and limited the acceptance of such designs.